Dielectrics having excellent high-frequency characteristics in the microwave region have been developed, and resonators employing these dielectrics are in use. However, there are cases where even a resonator employing a material having excellent dielectric characteristics cannot take full advantage of the excellent performances inherent in the dielectric material, depending on the method for fabricating the porcelain dielectric resonator employing the dielectric material as the resonator main body, the structure of the resonator, etc., and is hence reduced in dielectric characteristics rather than improved. For example, the currently extensively used porcelain dielectric resonators comprising a resonator main body fixed to the inside of a metal casing (electric-magnetic, shield case) through a supporting base have a problem that the porcelain dielectric resonator including the supporting base and the metal case as a whole may have a reduced value of Q.sub.u depending on the dielectric characteristics of the supporting base, the kind and thickness of the glazing material, adhesive, or the like used for bonding the resonator main body to the supporting base, and other factors.
It is therefore desirable to employ a supporting base which itself has a Q.sub.u as high as possible. Furthermore, since the supporting base functions also to transmit the heat generated by the resonator main body to the metal casing, it preferably further has excellent thermal conductivity. In alumina-based sintered materials, the thermal conductivity thereof tends to increase as the Al.sub.2 O.sub.3 content increases. A desirable supporting base is constituted of a material having an Al.sub.2 O.sub.3 content of 99% by weight or higher.
With respect to the production of general alumina-based sintered materials, techniques of using a lithium ingredient as a sintering aid are disclosed in, e.g., JP-A-60-21854, JP-A-60-151275, and JP-A-61-44757. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, the techniques disclosed in those references differ from the process of the present invention in the sintering aids used in combination with the lithium ingredient. The alumina porcelains described in JP-A-60-151275 and JP-A-61-44757 each has an Al.sub.2 O.sub.3 content of 96% by weight at the most. Furthermore, in JP-A-60-21854, there is especially no description concerning an improvement in the thermal conductivity of the sintered material, although the sintered material has a high Al.sub.2 O.sub.3 content.